Ford Expedition 2003 Owner's Manual page 155

Locks and Security
5. Within 10 seconds of removing the second coded key, insert the new
unprogrammed key (new key/valet key) into the ignition and turn the
ignition from 1 (OFF) to 3 (ON) (maintain ignition in 3 (ON) for at least
one second, but no more than ten seconds). This step will program your
new key to a coded key.
6. To program additional new unprogrammed key(s), repeat this
procedure from step 1.
If successful, the new coded key(s) will start the vehicle's engine and the
theft indicator will illuminate for three seconds and then go out.
If not successful, the new coded key(s) will not start the vehicle's engine
and the theft indicator will flash on and off and you may repeat steps 1
through 5. If failure repeats, bring your vehicle to your dealership to
have the new spare key(s) programmed.
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